Bug 1243398 - Move browser_newtab_undo.js to clipboard subsuite; r=jmaher

browser_newtab_undo.js is failing frequently on Windows.
In bug 1243103, similar failures were seen in browser_newtab_drag_drop_ext.js
until it was moved to the clipboard subsuite. Let's try to avoid failures
here the same way.

browser/base/content/test/newtab/browser.ini

@@ -49,5 +49,7 @@ support-files =
 [browser_newtab_sponsored_icon_click.js]
 skip-if = true # Bug 1314619
 [browser_newtab_undo.js]
+# temporary until determine why more intermittent on VM
+subsuite = clipboard
 [browser_newtab_unpin.js]
 [browser_newtab_update.js]